asyncio.Semaphore RuntimeError: Task got Future 附加到不同的循环 [英] asyncio.Semaphore RuntimeError: Task got Future attached to a different loop

问题描述

当我在 Python 3.7 中运行此代码时:

导入异步sem = asyncio.Semaphore(2)异步定义工作():与 sem 异步:打印('工作')等待 asyncio.sleep(1)异步定义主():等待 asyncio.gather(work(), work(), work())asyncio.run(main())

它因运行时错误而失败:

$ python3 demo.py在职的在职的回溯(最近一次调用最后一次):文件demo.py"，第 13 行，在 <module> 中asyncio.run(main())文件/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/runners.py"，第 43 行，运行中返回 loop.run_until_complete(main)文件/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/base_events.py"，第 584 行，在 run_until_complete 中返回 future.result()文件demo.py"，第 11 行，在主目录中等待 asyncio.gather(work(), work(), work())文件demo.py"，第 6 行，工作中与 sem 异步:文件/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/locks.py"，第 92 行，在 __aenter__等待 self.acquire()文件/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/locks.py"，第 474 行，在获取中等待未来RuntimeError: Task <Task pending coro=<work() running at demo.py:6>cb=[gather.<locals>._done_callback() at/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/tasks.py:664]>got Future <Future pending>附加到不同的循环

解决方案

这是因为 Semaphore 构造函数设置了它的 _loop 属性——在 asyncio/locks.py:

类信号量(_ContextManagerMixin):def __init__(self, value=1, *, loop=None):如果值<0:raise ValueError(信号量初始值必须> = 0")self._value = 值self._waiters = collections.deque()如果循环不是无:self._loop = 循环别的:self._loop = events.get_event_loop()

但是 asyncio.run() 在 asyncio/runners.py，它也在文档中提到:

def run(main, *, debug=False):如果 events._get_running_loop() 不是 None:引发运行时错误(无法从正在运行的事件循环中调用 asyncio.run()")如果不是 coroutines.iscoroutine(main):raise ValueError("a coroutine is expected, got {!r}".format(main))loop = events.new_event_loop()...

在 asyncio.run() 之外启动的

Semaphore 获取 asyncio 默认"循环，因此不能与使用 asyncio.run() 创建的事件循环一起使用.

解决方案

从 asyncio.run() 调用的代码中启动 Semaphore.您必须将它们传递到正确的位置，有更多的可能性如何做到这一点，例如，您可以使用 contextvars，但我只举一个最简单的例子:

导入异步异步定义工作(sem):与 sem 异步:打印('工作')等待 asyncio.sleep(1)异步定义主():sem = asyncio.Semaphore(2)等待 asyncio.gather(工作(sem)，工作(sem)，工作(sem))asyncio.run(main())

asyncio.Lock、asyncio.Event 和 asyncio.Condition 可能也存在同样的问题(和解决方案).>

When I run this code in Python 3.7:

import asyncio

sem = asyncio.Semaphore(2)

async def work():
    async with sem:
        print('working')
        await asyncio.sleep(1)

async def main():
    await asyncio.gather(work(), work(), work())

asyncio.run(main())

It fails with RuntimeError:

$ python3 demo.py
working
working
Traceback (most recent call last):
  File "demo.py", line 13, in <module>
    asyncio.run(main())
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/runners.py", line 43, in run
    return loop.run_until_complete(main)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/base_events.py", line 584, in run_until_complete
    return future.result()
  File "demo.py", line 11, in main
    await asyncio.gather(work(), work(), work())
  File "demo.py", line 6, in work
    async with sem:
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/locks.py", line 92, in __aenter__
    await self.acquire()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/locks.py", line 474, in acquire
    await fut
RuntimeError: Task <Task pending coro=<work() running at demo.py:6> cb=[gather.<locals>._done_callback() at /opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/tasks.py:664]> got Future <Future pending> attached to a different loop

解决方案

It's because Semaphore constructor sets its _loop attribute – in asyncio/locks.py:

class Semaphore(_ContextManagerMixin):

    def __init__(self, value=1, *, loop=None):
        if value < 0:
            raise ValueError("Semaphore initial value must be >= 0")
        self._value = value
        self._waiters = collections.deque()
        if loop is not None:
            self._loop = loop
        else:
            self._loop = events.get_event_loop()

But asyncio.run() starts a completely new loop – in asyncio/runners.py, it's also metioned in the documentation:

def run(main, *, debug=False):
    if events._get_running_loop() is not None:
        raise RuntimeError(
            "asyncio.run() cannot be called from a running event loop")

    if not coroutines.iscoroutine(main):
        raise ValueError("a coroutine was expected, got {!r}".format(main))

    loop = events.new_event_loop()
    ...

Semaphore initiated outside of asyncio.run() grabs the asyncio "default" loop and so cannot be used with the event loop created with asyncio.run().

Solution

Initiate Semaphore from code called by asyncio.run(). You will have to pass them to the right place, there are more possibilities how to do that, you can for example use contextvars, but I will just give the simplest example:

import asyncio

async def work(sem):
    async with sem:
        print('working')
        await asyncio.sleep(1)

async def main():
    sem = asyncio.Semaphore(2)
    await asyncio.gather(work(sem), work(sem), work(sem))

asyncio.run(main())

The same issue (and solution) is probably also with asyncio.Lock, asyncio.Event, and asyncio.Condition.

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